1. Technical Field
The technical field relates to a flexible circuit board, more particularly to a ultrafine circuit applicable in a flexible circuit board.
2. Related Field
Nowadays, most of the flexible circuit boards are fabricated by subtractive process, in which a photoresist (PR) is first coated onto a flexible copper clad laminate. Subsequent to the exposure and development processes, the required circuit pattern is transferred to the photoresist. The patterned photoresist is then used as a mask and wet etching is performed on the copper clad layer to form a circuit having the required pattern on the flexible substrate. However, affected by side etching, the copper circuit may have a trapezoid cross-sectional profile. The adverse effect of side etching is exacerbated with the increase in thickness of the copper foil layer or the fineness of the line width. In certain serious situations, an inverted triangle cross-sectional profile can happened to the copper circuit, and then make the subsequent configuration of devices become difficult, and poor signal transmission and reduction of the product yield. Therefore, for matching the trend of continuous thinning, lighting and down-sizing of electronic products, a further reduction of line width will require the application of a thinner copper layer. However, when the thickness of the copper layer is too thin (<8 μm), a parasitic copper rigidity—handability problem will make the manufacturing process become more difficult.
In order to improve the above mentioned handability of the thinner foil, wherein the application of the most popular subtractive process can be continued. A copper foil attached with carrier (hereafter named carrier-attached copper foil in abbreviation) has been developed through a support of carrier film with more rigidity and releasable capability after high temperature lamination processing. However, the highest operation temperature for the currently commercially available carrier-attached copper foil is only 300° C., which is still not enough for the high temperature and long duration (>360° C. for 1 hour) requirement of polyimide casting process. Moreover, even after being separated, the carrier foil is not usable again. Accordingly, by now, a carrier-attached copper foil is still expensive and unpopular. As a result, for the fabrication of LCD-COF (Chip On Flex), in which highly fine pattern is demanded, the industry generally uses the imported flexible copper clad laminate formed by sputtering process. A sputtering processed product, attributed to its necessity of passing a vacuum process, is not only expensive, but also parasitic with some undesirable properties, such as low peeling strength and low thermal stability, which would hinder the further development of fine pattern flexible circuit. Accordingly, the development of a carrier-attached copper foil remains an attention focus in the industry.